1. Field of the Invention
The present invention relates to the technical field of magnetic resonance imaging (MRI), and particularly to a method for correcting image distortion and system for diffusion-weighted echo planar imaging (DW-EPI) and magnetic resonance imaging equipment.
2. Description of the Prior Art
Diffusion imaging is a functional magnetic resonance imaging technology which has very high clinical importance, which can detect the biological properties of the diffusion of water molecules in human tissue and thus detect many pathologies which cannot be found by conventional medical imaging means. Diffusion imaging is usually implemented using an echo planar imaging (EPI) sequence; the imaging speed of the EPI sequence is very high but it is easily affected by various artifacts, especially image distortion.
Diffusion-weighted imaging (DWI) is an important imaging method of diffusion imaging, which is usually implemented using a DW-EPI sequence, and the DW-EPI sequence usually realizes diffusion weight by adding a diffusion gradient into the EPI sequence. The diffusion gradient of the DW-EPI sequence will introduce an eddy current field into the imaging space, and the additional magnetic field of eddy current field causes image distortion.
Since it is usually required to acquire several single images with different diffusion directions and diffusion weights (i.e. DW-EPI images) in diffusion-weighted imaging, and different diffusion gradients are used, causing different eddy current distributions, the distortion degrees and types of these single images are not completely identical. The appearance of the image distortion not only depends on the magnitude of the gradient, i.e. the diffusion weight, but also depends on the direction of the gradient. The size of the diffusion weight is usually determined through a so-called “b value”, and an image with the b value being zero usually does not have distortion, and an image with the b value not being zero has distortion. Hence, these single images with different b values and different diffusion directions do not overlap, and if these single images are combined with each other and calculated without correction to generate isotropic DWIs and apparent diffusion coefficient (ADC) graphs and the like required in diagnosis, this will cause blurs in the DWI and errors in the calculation of the ADC graphs, and the like.
Means for suppressing the eddy current field includes pre-enhancement of gradient coils and gradient pulses using self-shielding. Self-shielding coils include a main coil and a shielding coil, wherein the direction of the magnetic field generated by the shielding coil during imaging is opposite to that of the main coil, and as specially designed, this pair of coils can generate the required gradient magnetic field within an imaging area, and the magnetic field outside the imaging area is as approximate to zero as possible, which reduces the eddy current field essentially. The pre-enhancement technology is to counteract the influence of the eddy current field by changing the shape of the gradient waveform. These methods can be used to effectively reduce but cannot completely eliminate the influence of the eddy current field. The DW-EPI sequence is very sensitive to the eddy current field, and in order to improve the imaging quality, a special method is needed.